1. Field of the Invention
This invention relates to charging apparatus and, more particularly, to apparatus suitable for charging a nonaqueous secondary battery.
2. Description of the Prior Art
Among secondary batteries that can be repetitively charged and discharged, there are a secondary battery using an aqueous electrolyte and what is called a nonaqueous secondary battery using an organic electrolyte.
Among secondary batteries using an aqueous electrolyte, for instance, there are a lead acid battery, a nickel-cadmium battery, and the like. Among the nonaqueous secondary batteries, for instance, there are lithium system batteries such as a manganese-lithium battery, a carbon-lithium battery and the like, as shown in Japanese laid-open patent publication No. Sho 62-82670 and Japanese laid-open patent publication No. Sho 62-82671.
Each of the secondary batteries mentioned above has its own peculiar charging characteristics, and there are conventional charging systems respectively adapted to charge the different kinds of secondary batteries. Each of the conventional charging systems must have a construction that is suitable for the type of battery to be charged.
FIG. 1 shows the charging characteristics of a conventional nickel-cadmium battery for a constant charging current. The terminal voltage Vc of the battery is time-dependent and exhibits a mountain shape, and a full charging level Lf corresponds to a peak Pvc of the mountain shape. On the other hand, the charging current Ic always maintains a constant value, for instance one ampere.
FIG. 2 shows the charging characteristics of a conventional lead acid battery for a constant charging current. The terminal voltage Vc of the battery is time-dependent and exhibits a relatively rapid rise at the initial stage of the charging and a much slower rise after it reaches a predetermined voltage, so that it ultimately approaches a constant value. On the other hand, the charging current Ic maintains a nearly constant value, for example 1 A, throughout the charging process.
Characteristic graphs for conventional systems for charging nickel-cadmium batteries are shown in FIGS. 3 to 7. Among them, FIGS. 3 to 5 show characteristic graphs for systems for controlling the charging by detecting the terminal voltage Vc of the nickel-cadmium battery, and FIGS. 6 and 7 show characteristic graphs for systems for controlling the charging by detecting the temperature Tc of the nickel-cadmium battery.
According to the charging system of FIG. 3, a predetermined limit Lvc is set to the terminal voltage Vc, and the charging is stopped when it is detected that the terminal voltage Vc of the nickel-cadmium battery has reached the limit Lvc.
According to the charging system of FIG. 4, the charging is stopped when it is detected that the terminal voltage Vc of the nickel-cadmium battery has reached the peak Pvc.
The charging system of FIG. 5 relates to what is called a (-.DELTA.V) control system, wherein the charging is stopped when it is detected that the terminal voltage Vc of the nickel-cadmium battery has reached the peak Pvc and thereafter has decreased by a predetermined voltage .DELTA.V.
The charging system of FIG. 6 relates to a control system based on the temperature Tc of the nickel-cadmium battery. The battery temperature Tc at the end of the charging suddenly rises and becomes, for instance, about 40 to 50.degree. C. Therefore, the charging is stopped by employing a sensor to detect such an increase in the temperature.
The charging system of FIG. 7 relates to what is called a (.DELTA.T) control system. As the end of the charging process is approached, the battery temperature Tc begins to rise. When the increase in temperature reaches a predetermined value .DELTA.T, the charging is stopped.
For charging a lead acid battery, the charging characteristics of which are illustrated in FIG. 2, there are a conventional charging system (not shown) in which the charging is stopped when it is detected that the terminal voltage Vc of the secondary battery has reached a predetermined value and another conventional charging system (also not shown) in which the charging is stopped after a predetermined time as measured by a charging timer has elapsed.
In order to charge the nonaqueous secondary batteries mentioned above, one might first think of employing a conventional system designed for charging a secondary battery having an aqueous electrolyte. For instance, one might consider employing a system designed for charging a nickel-cadmium battery in order to charge a nonaqueous secondary battery. However, among the charging systems for the nickel-cadmium battery, the system shown in FIG. 3, in which the charging is carried out until the terminal voltage Vc reaches the limit Lvc, has the problem that the battery charge reaches about 80% at most; in other words, by means of this system the battery cannot be fully charged. On the other hand, the control system shown in FIG. 4 and the (-.DELTA.V) control system shown in FIG. 5 have the problem that they cannot be employed because the charging characteristics of the nonaqueous secondary battery do not show a mountain shape and do not have the peak Pvc required for operation of the systems of FIGS. 4 and 5.
Moreover, a control system based on the battery temperature Tc mentioned above also has a problem as a charging system for a nonaqueous secondary battery. For instance, if a system using an increase in the battery temperature Tc as an indication to limit the charging (FIG. 6) is used to charge a lithium system secondary battery, there is a danger of explosion.
Conventional charging systems for a lead acid battery, for instance, a system that sets the charging time using a timer, are also unsuitable for use in charging a nonaqueous secondary battery. The charging current Ic of a nonaqueous secondary battery changes substantially depending on the ambient temperature of the battery. Therefore, even if a constant charging time is set, the charging rate may not be stable and it is not always possible to be certain of charging the battery fully. On the other hand, a charging system based on the detection of the terminal voltage Vc of the secondary battery and used for charging a lead acid battery also presents a problem. Specifically, it is not always possible to be certain of charging the battery fully, because the performance of the secondary battery and the charging current Ic change substantially with changes in the ambient temperature of the battery even if the terminal voltage Vc of the secondary battery has reached the predetermined value.